HeaderMap has a capacity limit of 24576 headers. Adding any more headers triggers a panic when reserving more capacity.

I can't see any way to prevent the panic when handling incoming headers, except to hard-wire a check for this value into calling code.

It would be useful to expose this value as a public constant.

At the very least it should be documented. Currently HeaderMap::reserve says "Panics if the new allocation size overflows usize." This should be "Panics if the new allocation size is greater than 24576."